A bias circuit can be used for adjusting the operating point of an amplifier. Such a bias circuit can be formed as a current mirror circuit, for example, which biases the control terminal of an amplifier transistor. For a low noise amplifier, for example, it is desirable to reduce the noise from a bias circuit in order to ensure a high-quality performance of the low noise amplifier (LNA) with respect to important parameters or device parameters, e.g., the linearity of the amplifier, the compression point (P1dB) or the intermodulation point (IP3). The linearity of an amplifier, the compression point and the intermodulation point are determined among others by the bias current of an amplifier transistor and the way how the bias circuit, which delivers the bias current, is implemented. Such low noise amplifiers can, for example, be used in global positioning systems (GPS), wireless or mobile phone applications and in automotive applications of the bias circuit.
Conventionally, the noise of the bias circuit, for example, the noise of a current mirror transistor of the bias circuit, is blocked by a high-ohmic impedance.